Dwarf Fortress is probably the most complex game as of 2008, and it's definitely the most complex ASCII game. Okay, I kid you not, the Wiki is probably way over 200 MB, and doesn't have many pictures (and if it does, they aren't huge BMP's). It's huge. There's so much stuff in it. You can build most anything that comes to mind. And it's updated often.

It's easy to mod. I've done it. You can get a lot of cool things. And you can remove the innate alcohol dependency of the dwarves, if you need to. I do. I play a modded copy, and I have no qualms about cheating. I really don't care what others say. In a single-player game, I'll cheat all I want. And I'll be honest about it.

The Article Link leads to the full three dimensional view of the leetest Dwarf Fortress the majority of JU'ers will have seen. (Not many people here play DF, right?) It's gonna be hard to guess, so I'm gonna help you. You can download it and the mods I use (based off of 40d, IIRC). If you need it, you can get a mod for graphics here. I'd recommend it for novices, as it'll speed up the learning process incredibly. I'm also providing screenies in the DFG mod at the link above below.

The fortress below is called Waningink, and is, in my opinion, a very well designed fortress, making good use of surface area due to the aquifer below. It's also nigh inpenetrable.

In summary, Dwarf Fortress gets a full six @'s, and it deserves more. You can see the pictures below. If you look at both formats, you could get a better idea of what the fortress actually looks like. It's a mite confusing to just use the graphics one in my opinion. The screenshots are a mite small here, if you really wanna see them, download them.

Notice that the provided screenshots are made with DF's screenshot export tool, then converted to PNG, then to JPG. Quality loss is likely, and they're about 3200x2800, IIRC, so the pictures don't do any justice whatsoever. If you really want good diagrams, use the article link, these screenshots are inferior.